DMRS (Demodulation Reference Signal) or UE (User Equipment) specific reference signal is one of RS (Reference Signals) used in LTE-A (Long-Term Evolution-Advanced) (release-10, release-11, etc). DMRS is the same precoded as the data part in a LTE-A system, so it can provide the channel estimation for demodulation.
FIG. 1 is a schematic diagram showing an example of DMRS multiplexed. FIG. 1 shows a structure of RB (resource block) and DMRS. In FIG. 1, there is shown one RB. The abscissa axis (T) of the RB represents time (OFDM symbols), and its vertical axis (F) represents width of frequency band (sub-carriers). For the RB, the abscissa axis is divided into 14 sections, each of which forms an OFDM symbol in the vertical axis direction. The vertical axis is divided into 12 sections, each of which forms a sub-carrier in the abscissa axis direction. Each small block within the RB represents a RE (resource element), and all 12×14 REs of the RB form a sub-frame, which includes slot 1 and slot 2 along the abscissa axis direction. It is shown in FIG. 1 that the first three symbols from the left side of the RB are used as a control region. The remaining part of the RB is used to transmit data part, wherein the predetermined number of DMRSs are included in the RB, and allocated in different predetermined locations of the RB.
It is shown in FIG. 1 that DMRSs are multiplied with OCC (orthogonal cover code) and scrambling sequence, respectively. FIG. 1 gives DMRS examples of both rank 1 and rank 2 cases. For the rank 1 case, DMRS can use OCC [1,1] or OCC [1,−1] for its only one layer; for the rank 2 case, DMRS uses both OCC [1,1] and OCC [1,−1] which are used for one of layers of DMRS respectively. Because two OCCs are orthogonal to each other, for the rank 2 case, although two layers of DMRS occupy the same frequency/time REs, the orthogonality between the two OCCs still guarantees that two layers of DMRS are orthogonal to each other. It is noted that different layers of DMRS can be called different DMRS ports. For example, in FIG. 1, DMRS using OCC [1,1] can be called port 7 and DMRS using OCC [1,−1] can be called port 8. When only port 7 or port 8 is used, it is the rank 1 case; when both port 7 and port 8 are used, it is the rank 2 case.
On top of the OCC, there is a scrambling sequence [a1, a2, a3 . . . , b1, b2, b3 . . . ] initialized by a random seed. In FIG. 1, in one RE, port 7 and 8 use the same scrambling sequence. The term “same scrambling sequence” here means that the scrambling sequence is initialized by the same random seed. It is well known that a random seed in release-10 is calculated by the following equation (1),random seed=(└ns/2┘+1)·(2cell_id+1)·216+SCID  (1)wherein, ns represents the slot number (2 slots in FIG. 1 constitute one subframe), cell_id represents a transmission point ID (cell ID), and SCID is a binary value. As shown from the equation (1), in release-10, the DMRS random seed is decided by the slot number, transmission point ID and a binary value SCID. It is possible that in one transmission point, ports 7 and 8 can be configured with different values of SCID. In such case, port 7 and port 8 will use different scrambling sequences [a1, a2, a3 . . . ] and [b1, b2, b3 . . . ], for example. This mainly intends for MU (multi-user) operation and it will be discussed later.
Because DMRS is the base of demodulation at the receiver side, how to set the DMRS random seed is very important for different scenarios, which will be elaborated on the following.
CoMP Scenario
JT (Joint Transmission) is one technique of CoMP (Coordinate Multiple Points). FIG. 2 is a schematic diagram showing an exemplary JT scenario. It is shown in FIG. 2 that there are two transmission points (or cells) 1 and 2, both of which transmits to one UE (such as a mobile phone, etc) beams consisting of multiple RBs like the RB shown in FIG. 1. The two RBs in FIG. 2 are simplified representations of the RB in FIG. 1.
The principle of JT operation is illustrated in FIG. 2. In JT operation, different transmission points transmit identical data and DMRS to a UE, and the identical data and DMRS from different transmission points combine over the air. So the UE can enjoy the diversity gain from multiple transmission points. Therefore, for the JT operation, in order to correctly combine signals from the multiple transmission points, identical DMRS from the multiple transmission points are necessary; otherwise, DMRS from the multiple transmission points can not correctly be combined over the air. In this sense, the same random seed for initializing the scrambling sequence is necessary for JT operation.
However, it is likely that adjacent transmission points (cells) will have different transmission point IDs. For example, in release-10, adjacent transmission points (cells) may have different transmission point (cell) IDs. Because the parameter “cell_id” is involved in the random seed calculation as shown in the above equation (1), if transmission point IDs of different transmission points are different, their DMRS or DMRS scrambling sequences will also be different. Therefore, for JT operation, the key point is how to guarantee the same DMRS random seed for different transmission points having different transmission point IDs.
Non-CoMP Scenario
FIG. 3 is a schematic diagram showing an exemplary non-CoMP scenario. Unlike the case of FIG. 2, in FIG. 3, signals transmitted from two adjacent transmission points 1 and 2 are for different UEs, that is, UE 1 and UE2, respectively, i.e., UE1 receives the signals from the transmission point 1, and UE2 receives the signals from the transmission point 2. In non-CoMP operation, since the position of DMRS in a RB is fixed, DMRSs from adjacent transmission points may interfere with each other due to their overlapping in frequency and time resources. For example, as shown in FIG. 3, the signals transmitted from the transmission point 1 to UE 1 and the signals transmitted from the transmission point 2 to UE 2 overlap each other in frequency and time resources, thus their DMRSs interfere with each other (as indicated by dashed arrows in FIG. 3). Therefore, in this case, different DMRS scrambling sequences for adjacent transmission points are necessary to randomize such ICI (inter-cell interference).
However, it is likely that adjacent transmission points (cells) will have the same transmission point ID. For example, in release-11, adjacent transmission points may have the same transmission point ID. Because the parameter “cell_id” is involved in the DMRS random seed calculation as shown in the above equation (1), if adjacent transmission points have the same cell_id, DMRS scrambling sequences will be initialized by the same random seed, and ICI is generated to the DMRS of the adjacent transmission points. Therefore, for non-CoMP operation, the key point is how to guarantee different DMRS random seeds for adjacent transmission points having the same transmission point ID.
FIG. 4 is a schematic diagram showing a comparison between JT scenario and non-CoMP scenario. On the left side of FIG. 4, there is shown a JT scenario where adjacent transmission points have different transmission point IDs. In this case, DMRS random seeds for initializing DMRSs of transmission points 1 and 2 are respectively (└ns/2┘+1)·(2cell_id1+1)·216 and (└ns/2┘+1)·(2cell_id2+1)·216, both of which are obtained from the above equation (1) with a default SCID=0. Here, the parameter cell_id1 represents the transmission point ID of the transmission point 1 while the parameter cell_id2 represents the transmission point ID of the transmission point 2, and cell_id1 is unequal to cell_id2. In order to correctly combine DMRSs from the two transmission points 1 and 2 over the air, their DMRS random seeds are required to be identical especially in the case that their transmission point IDs are not the same. In conclusion, for the JT scenario, the same DMRS seed is necessary because DMRS combining over the air requires identical DMRS of JT transmission points.
On the right side of FIG. 4, there is shown a non-CoMP scenario where adjacent transmission points have the same transmission point ID, i.e. cell_id1. In this case, DMRS random seeds for initializing DMRSs of transmission points 1 and 2 respectively intended for UE1 and UE2 are both (└ns/2┘+1)·(2cell_id1+1)·216, which is obtained from the above equation (1) with a default SCID=0. In order to randomize ICI (as indicated by dashed arrows in FIG. 4) between DMRS from the two transmission points 1 and 2, their DMRS scrambling sequences are required to be different especially in the case that their transmission point IDs are the same. In conclusion, for the non-CoMP, different DMRS seeds are necessary to randomize the ICI from overlapped DMRSs.
For the JT and non-CoMP scenarios as described above, it is concluded that these two scenarios have conflict requirements on DMRS random seed: the JT operation requires the same DMRS random seed while the non-CoMP operation requires different DMRS random seeds.
MU-MIMO Scenario
In addition to the above two scenarios, MU-MIMO (Multi-user Multiple Input-Multiple Output) scenario needs to be considered. FIG. 5 is a schematic diagram showing an exemplary MU-MIMO scenario. The principle of MU-MIMO operation is illustrated in FIG. 5. For MU-MIMO operation, two or more UEs are assigned to the shared frequency/time radio resource. It is shown in FIG. 5 that there are one transmission point and two UEs, that is, UE1 and UE2, both of which share the same frequency/time resource. Because the positions of DMRSs overlap, if one UE such as UE1 can estimate the channel of interfering UE such as UE2 from DMRS, then this UE such as UE1 can cancel the MU interference (as indicated by dashed arrows in FIG. 5) on its side.
Such UE side interference cancellation depends on whether or not a UE can blindly detect the DMRS of interfering UE. FIG. 6 is a schematic diagram showing an exemplary blind detection. The principle of the blind detection is shown in FIG. 6. The freedom of DMRS inside one transmission point (cell) is from 2 aspects: one is from DMRS random seed by setting SCID=0 or 1; the other is from OCC by setting OCC as [1,1] or [1,−1] (or equivalently choosing DMRS port 7 or 8). Specifically, as shown in FIG. 6, assuming that the transmission point ID of the transmission point is cell_id1, it can be obtained that Seed0=(└ns/2┘+1)·(2cell_id1+1)·216 and Seed1=(└ns/2┘+1)·(2cell_id1+1)·216+1 from the above equation (1) with SCID being 0 or 1. Combination of two different DMRS random seeds and two OCCs results in four different DMRSs: Seed0, OCC [1, 1]; Seed0, OCC [1, −1]; Seed1, OCC [1, 1]; Seed1, OCC [1, −1]. Each of the four different DMRSs is used for one UE. For example, as shown in FIG. 6, the DMRS with Seed0 and OCC [1, 1] is used for UE0; the DMRS with Seed0 and OCC [1, −1] is used for UE1; the DMRS with Seed1 and OCC [1, 1] is used for UE2; and the DMRS with Seed1 and OCC [1, −1] is used for UE3. Because there are totally 4 dimensions of DMRS, one UE can blindly detect whether or not there are one or more of other three UEs' DMRSs on the shared resource. This blind detection is feasible since the detection space is limited within 4 dimensions of DMRS. It is not difficult to find that DMRS random seed design largely affects such blind detection. The release-10 DMRS random seed design enables such blind detection, which should be considered as one design aspect for further improvement on DMRS random seed.